A digitizer tablet has been designed as an I/O device for inputting information into a data processing system. The tablet comprises a transflective LCD (liquid crystal display) and a magnetic digitizer capturing a user's handwriting input. A planar member supports a microprocessor module and a set of support chip modules such as used in the IBM Model L40 laptop computer. The microprocessor provides the necessary processing power required to execute complicated pen-based operating systems and required functions. The tablet provides versatile I/O options with serial, parallel, floppy scsi, keyboard, and cellular transceiver connectors all residing on the planar. The planar member also has a connector that supports either 4 Mb or 8 Mb DRAM cards to serve as the system memory. Furthermore, the system features a 9600 baud DSP modem and a 20 Mb solid state hardfile with removable "smart" cards.
All of this function adds up to a significant amount of power being dissipated within the housing. Power conversion is handled through copper Kells card technology. The tablet in worst case conditions can dissipate as much as 14 watts within its covers. Customer requirements for small size, low weight, long battery life, and a quiet tablet ruled out the use of a DC controlled fan to provide cooling. An innovative approach had to be used to remove the heat from the internal components such that the independent chips would meet their reliability requirements. This approach required that the "hot" or high powered components conduct their heat out through the base.
The closest prior art known to us includes various examples of cooling apparatus. IBM Technical Disclosure Bulletin, Vol. 28, No. 4, September 1985, page 1490, discloses a THERMALLY CONDUCTIVE ELASTOMER FOR COOLING in which a printed circuit board with components thereon is placed on top of a thermally conductive elastomeric putty which, in turn, is placed on top of a flat base having cooling fins depending from the lower surface thereof. The putty appears to extend substantially completely across the bottom of the planar member and the base.
U.S. Pat. No. 4,571,456--D. C. Paulsen et al discloses a PORTABLE COMPUTER having a plurality of heat generating modules mounted on the bottom surface of a planar member. The modules are mounted so as to provide a small air gap across which heat is transferred into the base. The base acts as a heat sink for dissipating the heat but the base has no special shape facilitating heat dissipation.
U.S. Pat. No. 4,602,314--Broadbent, discloses a HEAT CONDUCTION MECHANISM FOR SEMICONDUCTOR DEVICES. A heat sink is spaced above a semiconductor device and a resilient, deformable body having a thermally conductive outer layer is placed between the device and the heat sink. Heat is conducted by such layer around the deformable body from the device to the heat sink. The devices are arranged in rows and the body and outer is formed in a strip and extends along a row of devices.
U.S. Pat. No. 4,029,999--Newmann et al discloses a THERMALLY CONDUCTING ELASTOMERIC DEVICE that includes an elastomeric pad provided with molded indentations conforming to all the components on a printed circuit board. The pad conducts heat from such components to a heat sink placed above the pad.
However, such prior art may not be useful for a digitizer tablet. A digitizer tablet is intended to be handled by a user in a manner similar to how a writing pad or tablet is used, whereby the tablet can be placed on a desk or a table or on the user's lap. The tablet may be hand held or cradled in one of the user's arms while the free hand "writes" on the tablet. Since there are many different ways the user might come in contact with the tablet during use, a design requirement, in addition to that of being able to effectively dissipate power and heat energy, is that the tablet be constructed to preclude the user from contacting any hot spot which could burn the user or produce user discomfort. In the prior art discussed above, this problem is not discussed, acknowledged or dealt with.